The present invention relates to a digital path monitoring method whereby bit errors caused in a multiplexer section and a demultiplexer section included in a digital signal multiplex and communication system can be detected, a multiplexer to which this monitoring method has been applied, and a communication system formed by using this multiplexer.
In a digital multiplex transmission system, it is necessary to always monitor the digital path and occurrence of bit error, signal input interruption or from alignment error between opposing multiplexers in order to raise the system reliability. In a conventional monitoring method as described in "Shisetsu", Vol. 31, No. 8, p. 120, for example, parity bit is given on the sending side by counting "1" bits included in each frame of a multiplexed signal, and bit errors are detected at the receiving side by performing similar counting and parity check.
The monitoring method described in the aforementioned paper is capable of detecting transmission errors contained in multiplexed digital signals, but is not capable of detecting bit errors caused in the multiplexing and demultiplexing process itself. As methods for detecting such bit errors caused in the multiplex or demultiplex section, the "In-out check" scheme described in 1979 National Convention Record of the Institute of Electronics and Communication Engineers of Japan, No. 2036, for example, and a scheme as described in Al Geigel, "Monitoring the Performance of Digital Multiplex Circuits" Bell Laboratories Record, August 1971 can be mentioned. In these schemes, however, a higher-rate group signal to be monitored is demultiplexed into lower-rate group signals, and each of these demultiplexed signals is compared with the input lower-rate signal in the multiplex circuit bit by bit. Therefore, a demultiplex circuit and a comparison circuit must be newly added, resulting in a problem of increased hardware.
An object of the present invention is to provide a digital path monitoring method, for a multiplexer using a pulse stuffing technique and a communication system including an asynchronous multiplex function using a simple hardware configuration.